fix: address Cubic findings for runtime fallback child sessions

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/runtime-fallback/last-user-retry-parts.ts

@@ -1,14 +1,22 @@
-type SessionMessagesResponse = {
-  data?: Array<{
-    info?: Record<string, unknown>
-    parts?: Array<{ type?: string; text?: string }>
-  }>
+type MessageItem = {
+  info?: Record<string, unknown>
+  parts?: Array<{ type?: string; text?: string }>
+}
+
+function extractMessages(messagesResponse: unknown): MessageItem[] | undefined {
+  if (messagesResponse == null) return undefined
+  if (Array.isArray(messagesResponse)) return messagesResponse as MessageItem[]
+  if (typeof messagesResponse === "object") {
+    const data = (messagesResponse as Record<string, unknown>).data
+    if (Array.isArray(data)) return data as MessageItem[]
+  }
+  return undefined
 }
 
 export function getLastUserRetryParts(
   messagesResponse: unknown,
 ): Array<{ type: "text"; text: string }> {
-  const messages = (messagesResponse as SessionMessagesResponse).data
+  const messages = extractMessages(messagesResponse)
   const lastUserMessage = messages?.filter((message) => message.info?.role === "user").pop()
   const lastUserParts =
     lastUserMessage?.parts